We are planning a major addition to our 1950’s bungalow which has previously had the crawlspace dug out to create a low clearance (6ft) basement under part of the house.  In order to leave as much backyard as possible, we’d like to go with a full basement for the new addition, but I’m not sure the best way to way to accomplish this. 

The existing foundation is 9″ concrete with no footing, the house is well supported and not showing any signs of sinking or cracking.  I know enough to realize that I need to talk to so professionals in the area regarding the new foundation and a structucal engineer, but I’d like to educate myself about the potential options for a new foundation for the addition prior to having that conversation.  My biggest concern is that the intitial excavation will potentially undermine the existing wall, if not done very carefully by the pros.

From what I’ve read online I can see 2 options either:

1 – Underpin the existing wall, extending it down to the new basement addition depth and tie into the existing wall.

2 – Temporarly support the rear of the house and completely remove the existing wall and replace with the new wall and new sill tieing back into the floor joist. 

Any feedback regarding the feasibility and or potential costs of these 2 methods from the forum would be greatly appreciated! Or is there a better way do this?